Which is the number of atoms in 4.5 moles of hg, correctly written in scientific notation with the correct number of significant figures?

Avogadro's number represents the number of units in one mole of any substance. This has the value of 6.022 x 10^23 units / mole. This number can be used to convert the number of atoms or molecules into number of moles. 

4.5 moles ( 6.022x10^23 atoms / 1 mol ) = 2.7x10^24 atoms Hg.

The dispersed phase is liquid and dispersed medium in milk is liquid